A known impregnated cathode is formed such that a porous tungsten base which is formed from tungsten powder, is impregnated with electron emitting material. The cathode is received in a storing bath made of high melting point metal and the electron emitting face of the cathode is coated with platinoid elements. It is known that such cathode has the advantages of longer life and better quality than those of a conventional oxide cathode material made of carbonate.
To obtain such a desirable cathode structure, the porous tungsten base is made by being pressed or sintered and impregnated with an electron emitting material having barium as a main element to soak into the porous space formed in the electron emitting material. A part of one side which forms the electron emitting face in the tungsten base is removed, and an impregnated portion remains, and then after the tungsten base is received in a molybdenum receiver, the exposed face is coated from the bath with platinoid elements.
The cathode structure formed as described above emits electrons such that when the porous tungsten is heated at 1000.degree.-1200.degree. C. by an associated heater, barium oxide (BaO), a barium type electron emitting material impregnated in the tungsten, is separated and barium Ba is reduced to free barium, which spreads over the surface of the cathode.
U.S. Pat. No. 4,379,979 issued to Thomas et. al. Apr. 12, 1983 and U.S. Pat. No. 4,417,173 issued to Tuck et. al. Nov. 22, 1983, disclose improved examples related to the above-described impregnated cathode structure.
In the interim period, color cathode ray tubes, especially cathode ray tubes having high resolution, improve the screen visibility by minimizing the diameter of the beam spot formed on screen.
Generally, to reduce the diameter of beam spot, a material having high current density is used in the impregnated cathode. Further, the aperture diameter of a control electrode(first grid) is minimized from 0.6 mm O to 0.3 mm O. However, as aperture diameter of the control electrode becomes small, the charge which is applied to the cathode should be increased. At that time, electron emission from the cathode is rapidly increased beyond that necessary, so the useful life of the cathode is noticeably shortened.